In the early 1960's, British Scientist Bangham and others found that phosphatide may form small multilayered capsules while dispersing in water, and discovered that each layer of bimolecular lipids in a multilayer cyst is separated by water, the thickness of each layer being about 40 .ANG.. This kind of capsule formed of microparticles, having a structure similar to a biomembrane, is termed a liposome.
Since liposomes comprise a hydrophilic and/or lipophilic "small room", these "small rooms" may envelope molecules and ions which are soluble in water or lipid. Due to these specific characteristics, liposomes can be useful as a carrier, especially for pharmaceuticals. Further, liposomes may alter the mechanism of pharmaceutical metabolism and selectively transport the pharmaceutical to the "target", then release the pharmaceutical at the proper part of tissues to be cured. Thus, toxicity to normal cells can be reduced and the curative effect, i.e. attack on deleterious cells, can be greatly enhanced. In addition, there is another specific property in that the pharmaceutical enveloped by the liposome may be slowly released into local sites whether entering into blood circulation or combining with cells and tissues, even entering into cells through pinocytosis. With the slow release mechanism of liposomes, the half-life of the effective pharmaceutical is prolonged and the therapeutic effect is obviously improved. Furthermore, liposomes are made from natural phosphatides and cholesterols, having low toxicity, free from immunogens and with suitable bio-compatibility and bio-degradable properties. Recently, liposome engineering has made progress and the application of liposomes as a carrier for pharmaceuticals has been promoted.
In 1971, British scientist Ryman and others suggested that liposomes could be used as a carrier for enzymes or pharmaceuticals. Based on research of biomembrane theory, Ryman started to envelop enzymes or pharmaceuticals into liposomes, in order to protect for the active substances or pharmaceutical from destruction in blood circulation and to selectively attack the deleterious cells of the target tissue.
Chinese Canadian scientist Chang Ming-Su has researched liposome carriers of pharmaceuticals for therapic usage. In 1985, a famous Chinese Professor, Gu Xue-qiu, got the highest honor at the Fourth International Cancer of Lungs Conference for reporting the success, in the 1980's, of two kinds of polyphase liposomes as anti-cancer pharmaceutical carriers in combination with other chemical drugs to combat cancer.
Polyphase liposomes are new forms of anti-cancer pharmaceutical carriers of a polyphase-dispersing system. By enveloping some anti-cancer pharmaceutical into the carrier, and administering it into the blood circulation of the patient by intravenous injection, the polyphase liposome can exactly hit the target cancer tissue as a so-called "ultra-micro missile". The polyphase liposome is of a milky-white emulsion. Several kinds of emulsive intravenous injections which have been developed are composed of ultra-micro particles of a drug carrier, having a lymph system orientation, some of them penetrating the lysosome through phagocytosis of the phagocyte of dicty endothelial system, upon which the pharmaceutical is released upon digesting. Others release the pharmaceutical upon digesting by means of a fusion function, namely, due to the liposome membrane being similar to that of the cell membrane, the polyphase liposome fuses into the cell, thus maintaining a rather high concentration of the drug in the target tissue Such an orientation significantly reduces the toxicity and attack of anti-cancer pharmaceuticals on normal cells. As such, the therapic effect of the anti-cancer pharmaceutical will be enhanced while the toxicity to the normal cells is reduced.
In general, a liposome may be deemed to be an artificial membrane of cell, having an enclosed spheroidal structure (i.e., a "smallroom") with a diameter of about 300-2000 .ANG. and a maximum diameter of about 5.mu.. Liposomes having relatively large diameters cannot penetrate into the cytoplasm directly. In such cases, it is necessary for the liposome to be phagocytized by the cell.
Up to the present, there have been in the prior art liposomes which mostly comprise phosphatide as a skeleton material and additives. Phosphatide is an amphoteric compound possessing hydrophilic and lipophilic radicals, including natural phosphatide (lecithin and soy-bean) and synthetic phosphatide (such as phosphatidyl choline, dipalmitol phosphatidyl choline and distearyl phosphatidyl choline). These phosphatides provide two hydrophilic chains. They form liposomes of bimolecular layers in water, no matter how the structure of the hydrophilic radical is. The additives used in the prior art are, for example, cholesterol, octadecamine and phosphatidate, etc. Although cholesterol is useful for regulating the flowability and permeability of a bimolecular layer, cholesterol is not good for human beings. Octadecamine and phosphatidate may be used to alter the surface electrical charges of a liposome. The components of a polyphase liposome may be phosphatide, oleic acid, cholesterol and nonionic surfactants such as PVP (polyvinylpyrrolidone).
Much research work has been directed to liposomes as drug carriers, as models of bio-membranes, and methods of preparation. Polyphase liposomes are formed after the phosphatides contact with water. Formation is due to the action if its polar group and hydrophobic group which lead to the formation of poly bimolecular layers of a closed type spherical structure. The water layer is laid between the bimolecular layers as the water-soluble drugs are enveloped into it, the liposoluble pharmaceutical being enveloped in the bimolecular layers. Many factors as surface characteristics, particle sizes, differences in forms, surface electrical charges of the liposome can effect the stability in vivo and the percentages of enveloped pharmaceutical. The factors depend upon the components of phosphatides and methods of preparation.
Chemical properties of phosphatide with unsaturated fatty acid chain, such as of lecithin and soya bean lecithin are sometimes not sufficiently stable. Phosphatide is susceptible to oxidation and hydrolysis. Thus, peroxides, propanediol and lysophosphatide are produced. The oxidation of lecithin will subject the membrane formed to decreased flowability and increased stability and negative electrical charge conditions. Thus leakage of drugs will be promoted so that retaining of drugs will become less and the liposome will be easily aggregated and precipitated, thereby producing toxicity. Therefore, it has been suggested that lecithin as a membrane material should have high purity and an oxidation index of less than 0.2.
In preparation of liposomes it is a difficult problem to envelope a large quantity of drug. For example, where the liposolubility and aqueous solubility of the pharmaceutical are both low, the envelopment quantity of the drug will be less. And where the molecule of the drug is small and easily subject to percolation, the envelopment quantity of the drug will be weaker.
In the prior art, generally speaking, it has been known that a liposome or a polyphase liposome possesses advantages as a carrier for pharmaceuticals. For example, they may enter into cells as a carrier of a pharmaceutical, enabling the pharmaceutical to maintain its therapeutic effect for a prolonged period of time since the pharmaceutical enveloped by the carrier will avoid destruction in the blood circulation. During preparation, pharmaceuticals which may be hydrophilic or lipophilic can be enveloped. They may be formulated as "water in oil" or "oil in water" type emulsions, so that they may be phagocytized by cells through phagocytosis when they touch the surface of the cells. Furthermore, due to their lymph system orientation and selectivity, the amount of their entering into the lymphoglandulae, which is full of dicty endothelium cells, will be large, etc. However, in the data reported in the prior art there are some shortcomings at the present time in liposome or polyphase liposome, such as that their particle sizes formed are relatively large (about ten times bigger than the CHML of present invention), and that the fact they might cause systemic capillary circulation obstacle when they enter the vein. Further, they should be phagocytized by cells to enter them, thus there must be an energy consumption in the cells. In addition, their idiosyncratic phagocytosis by deleterious cells system in cancer cells is not so strong that they shall be often phagocytized by normal cells, causing the normal cells to be poisoned and the bioeffect of the pharmaceutical to be reduced. Also, their promotion of immunocytes of human beings is seldom reported in the prior art. They could not envelop two or more phases of pharmaceutical in the same layer. Finally, they may contain cholesterol and polyvinyl pyrrolidone (PVP) which exert unhealthy effects to human beings, etc.